Fire suppression systems are commonly used in commercial buildings for extinguishing fires. In one type of fire suppression system, a jet of liquid fire extinguishing agent, commonly water from a water supply tank, is injected into a high velocity stream of pressurized inert gas from an inert gas storage tank as the inert gas is passing through a delivery pipe communicating with a network of distribution pipes. Upon interaction of the high velocity stream of inert gas with the water jet, the water droplets in the water jet are atomized into a mist of very small or minute droplets, typically having a median droplet size ranging between 5 and 60 micrometers, thereby forming a two-phase mixture of water mist droplets entrained in and carried by the inert gas stream. This two-phase mixture is distributed via the network of distribution pipes to a plurality of spray nozzles mounted to the distal ends of the respective distribution pipes. The spray nozzles spread the water mist droplets and inert gas over a desired area to in effect flood that area with water mist droplets and inert gas for extinguishing a fire in the protected volume.
The inert gas commonly used in conventional inert gas fire suppression systems is nitrogen, but argon, neon, helium or other chemically non-reactive gas, or mixtures of any two or more of these gases may be used. The inert gas suppresses fire within the protected volume by increasing the heat capacity per mole of oxygen and diluting the oxygen content within the protected area. Additionally, the water mist droplets enhance fire suppression by also raising the overall heat capacity of the atmosphere within the protected volume. Due to the presence of the water droplets, the two-phase mixture of water mist droplets and inert gas has a higher overall heat capacity than the inert gas alone. Consequently, the two-phase mixture of water mist droplets and inert gas will more effectively absorb heat from the flame sheath to the point that the temperature of the gas within the vicinity of the flame sheath drops below a threshold temperature below which combustion can not be sustained, for example below 1800 degrees C.
International Patent Application No. PCT/GB02/01495, published as International Publication WO02/078788, for example, discloses a water and inert gas fire and explosion suppression system of the type hereinbefore described.
A potential concern associated with such systems is freezing of the water droplets as the two-phase mixture passes through the network of distribution pipes. As the inert gas passes from the supply cylinders to the spray nozzles, the inert gas expands as the pressure drops from the supply pressure of 200 to 300 bars to atmospheric pressure. This adiabatic expansion of the inert gas causes a cooling of the inert gas that may generate temperatures in the range of −60 degrees C. to −100 degrees C. Such extreme temperatures may result in a significant amount of the water droplets freezing as they traverse the network of distribution pipes. Since frozen water droplets attach to the pipe walls they will not take part in extinguishing a fire, if a sufficient degree of freezing of water droplets occurs, the fire suppression effectiveness of the system will be degraded.